In recent years, lithium ion batteries that feature light weight and high charge capacity have become widely used, as secondary batteries in cellular phones, notebook computers, electric cars and the like whose size and weight have been reduced and whose performance has been improved. In applications such as battery cars and electric tools, one problem is that high-current load properties is insufficient. As measures to address this problem, efforts have been made to reduce electrode impedance, more specifically, efforts have been focused on improving rate characteristics. In addition, volume change accompanying charge and discharge also leads to an increase in electrode impedance and the impedance between electrode materials and current collectors, and therefore, reducing the amount of volume change is one approach to achieve a reduction in impedance.
As a study of resistance reduction, Patent Literature 1 discloses a negative electrode for a lithium ion battery including a negative electrode active material, a conductive carbon material containing carbon fibers and a binder, wherein the carbon fibers are present in a state of not forming aggregates having a size of 10 μm or more. In Patent Literature 1, a mixture obtained by dry-mixing a negative electrode active material and carbon fibers, and polyvinylidene fluoride as a binder are stirred and mixed to obtain a negative electrode slurry.
Patent Literature 2 discloses a lithium ion battery including a negative electrode obtained by mixing carbon nanohorns with a graphite material.
Patent Literature 3 discloses a negative electrode material obtained by mixing carbon nanotubes and a thermoplastic resin and heating the mixture in an inert gas to coat the carbon nanotubes with carbon by pyrolysis.